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To add some sort of backend to your Stores, you can make use of fritz2’s repositories. fritz2 offers implementations of the repository-interfaces for two types of backends:

  • LocalStorage
  • REST


When defining the data class you want to share via Repository, you must define a Serializer for it. You can of course use kotlinx-serialization for this. Annotate your data class (here Person) with @Serializable and then write the following serializer for your class:

object PersonSerializer : Serializer<Person, String> {
    override fun read(msg: String): Person = Json.decodeFromString(Person.serializer(), msg)
    override fun readList(msg: String): List<Person> = Json.decodeFromString(ListSerializer(Person.serializer()), msg)
    override fun write(item: Person): String = Json.encodeToString(ToDo.serializer(), item)
    override fun writeList(items: List<Person>): String = Json.encodeToString(ListSerializer(Person.serializer()), items)


Working with repositories requires the definition of a Resource:

val personResource = Resource(
    Person::_id, // function to extract the unique id from a given instance
    PersonSerializer, // implementation of Serializer interface to write and read the entity
    Person() // definition of an empty entity (to reset an edit form, e.g.)


We differentiate two kinds of repositories:

  • an EntityRepository deals with one single entity of a given type. It offers the usual CRUD-methods:
    • load(entity, id)
    • addOrUpdate(entity)
    • delete(entity)
  • a QueryRepository deals with a List of instances of a given type. It offers the following methods to query or manipulate the content of the repository:
    • query(entities, query)
    • addOrUpdate(entities, entity)
    • updateMany(entities, entitiesToUpdate)
    • delete(entities, id(s))


To connect a Store with a REST-backend for example, just add the repository-service and use its methods in your handler:

// use the defined resource from above
val entityStore = object : RootStore<Person>(personResource.emptyEntity) {

    private val rest = restEntity(personResource, "https://your_url")

    val load = handle<String> { person, id ->
        rest.load(person, id)

    val saveOrUpdate = handle { rest.saveOrUpdate(it) }

    val delete = handle { rest.delete(it) }
    init {

syncBy automatically calls the given Handler on each update of your Store.


When creating a QueryRepository, you can define a type describing the queries which are done by this repository, and a function for executing the query defined by a given instance of this query-type.

data class PersonQuery(val namePrefix: String? = null)

val queryStore = object : RootStore<List<Person>>(emptyList()) {
    val rest = localStorageQuery<Person, String, PersonQuery>(personResource, "your prefix") { entities, query ->
        if (query.namePrefix != null) entities.filter { }   
        else entities

    val query = handle(execute = rest::query)
    val delete = handle<String>(execute = rest::delete)

    init {

Of course the receiver and result of this function depend on the concrete implementation you use. For a RestQuery, you have to build and fire the request according to your query-object. If you do not specify a function, all entities of the defined Resource will be returned.

val queryStore = restQuery<Person, String, PersonQuery>(personResource, "your url") { query ->
    get(query.namePrefix?.let {"?name=$it"}.orEmpty())

For more information about the remote parameter of restQuery, please refer to the docs section HTTP-Calls.


You can see repositories of all types in action at

Distributed by a MIT license.